DNA cleaving enzymes associated with methyltransferases are widely present in the prokaryotic genomes. The DNA cleaving enzymes typically consist of restriction endonucleases, which protect host cells from invading DNA (e.g., bacteriophages) by cleaving DNA at defined sites, and DNA methyltransferases, which protect host DNA from being degraded by methylating a specific base within the restriction endonuclease sites (Roberts, et al. Nucleic Acids Res 35: D269-270 (2007)). Hence, these restriction endonucleases are termed methylation-sensitive.
While modified bases in prokaryotes and phage DNA play a role in protecting the genome against cleavage by restriction endonucleases, methylated cytosine (m5C) is involved in gene expression of the mammalian genome. Techniques for identifying methylated DNA are cumbersome and experimentally difficult to implement in a reproducible fashion. Two approaches are commonly used. One involves the use of restriction enzymes like HpaII and MspI which are differently sensitive to cytosine methylation. For example, HpaII endonuclease is blocked by methylation of either of the two cytosines within the CCGG recognition site, but its isoschozimer, MspI, is blocked only when the outer C is methylated. It will cleave DNA when the inner cytosine is modified. The second method involves bisulfite modification of the unmethylated cytosine residues followed by selective amplification and sequencing of the remaining DNA. In this method, methylated cytosines are resistant to the treatment. This method is not easy to optimize and involves a complicated chemical modification step followed by amplification using a complicated set of primers. The method is widely used in the absence of simpler alternative approaches.